Move ResourceLoader to use WebURLLoader directly.

Remove the ResourceHandle layer, merging its (minimal) logic into ResourceLoader.

Merge ResourceHandleTypes.h into ResourceLoaderOptions.h, as there's no reason
for the separation with the removal of the ResourceHandle layer.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19881006

git-svn-id: svn://svn.chromium.org/blink/trunk@154772 bbb929c8-8fbe-4397-9dbb-9b2b20218538